Abstract

Background

We identify DNA transposons from the completed draft genome sequence of Daphnia pulex, a cyclically parthenogenetic, aquatic microcrustacean of the class Branchiopoda.
In addition, we experimentally quantify the abundance of six DNA transposon families
in mutation-accumulation lines in which sex is either promoted or prohibited in order
to better understand the role of recombination in transposon proliferation.

Results

We identified 55 families belonging to 10 of the known superfamilies of DNA transposons
in the genome of D. pulex. DNA transposons constitute approximately 0.7% of the genome. We characterized each
family and, in many cases, identified elements capable of activity in the genome.
Based on assays of six putatively active element families in mutation-accumulation
lines, we compared DNA transposon abundance in lines where sex was either promoted
or prohibited. We find the major difference in abundance in sexuals relative to asexuals
in lab-reared lines is explained by independent assortment of heterozygotes in lineages
where sex has occurred.

Conclusions

Our examination of the duality of sex as a mechanism for both the spread and elimination
of DNA transposons in the genome reveals that independent assortment of chromosomes
leads to significant copy loss in lineages undergoing sex. Although this advantage
may offset the so-called 'two fold cost of sex' in the short-term, if insertions become
homozygous at specific loci due to recombination, the advantage of sex may be decreased
over long time periods. Given these results, we discuss the potential effects of sex
on the dynamics of DNA transposons in natural populations of D. pulex.